The contamination of soil and ground water with toxic substances poses health problems to both animals and humans. A particular example of such contamination is the presence of aromatic amines in soil and groundwater above acceptable toxicity levels. Accordingly, in areas where such contamination has occurred, remediation and recovery of such compounds is necessary in order for land to be suitable for agriculture, forestry or recreation.
In particular, activities carried out at former Soviet military bases in Latvia have resulted in the discharge of various toxic substances into the environment. Since the abandonment of these bases, local residents have been able to access these lands thereby exposing them to the risk of exposure to the toxic substances. Large scale clean-up operations are required in order to return these lands to safe use.
Soil and water samples taken from these lands revealed high levels of xylidine-based missile fuel SAMIN and various oil products. Oil contamination is certain areas reached 6,000-7,000 ppm while the xylidine contamination reached several thousand ppm and as high as 20,000 ppm in particular locations. Xylidines have been found to be toxic at levels as low 2 ppm upon inhalation or skin contact. Accordingly, a high degree of remediation is required in order to return contaminated land to a safe level.
Furthermore, as xylidine compounds are valuable compounds in a variety of industrial processes, there has been a need for non-destructive remediation technologies in which the xylidine compounds may be recovered for re-use.
A variety of soil treatment technologies exist for the remediation of contaminated soil such as soil washing, solvent extraction and low temperature thermal desorption.
Soil washing is a physical treatment method in which contaminants are removed from the soil by solubilizing them, or suspending them in a fluid such as water with or without surfactants or detergents. The soil is separated from the washing fluid with the contaminants, fine soil particles and soluble components of the soil staying in the washing fluid. Ultimately the contaminants are concentrated in the washing fluid and the "cleaned" soil is tested to ensure the contaminants have reached target levels and then returned to its original site. In some cases, soil washing alone can reduce the contaminant concentrations to acceptable levels and therefore serve as a stand-alone technology. In other cases, it can be a cost-effective pre-processing step in reducing the quantity of material to be processed. It is important to note that this process does not destroy but rather concentrates the contaminants for further processing. Although this treatment method is suitable for a wide range of contaminant problems, generally it is most effective on coarse material.
Solvent extraction is differentiated from soil washing because it utilizes organic solvents or critical fluids to remove hydrophobic contaminants from the soil. The extracting fluid is then separated from the soil by a physical method such as filtration. The soil may require additional treatment to remove any residual extracting fluid. The extracting fluid may then be treated and recycled back into the process. The effectiveness of this process is dependent upon the nature of the contaminant, soil type and extracting agent used. Several methods of enhancing solvent extraction have been developed.
Low temperature desorption is a process by which either direct or indirect heating is used to raise the temperature of a contaminated soil to volatilize the organic contaminants and water into an exhaust gas. The contaminants in the exhaust gas are either destroyed in an afterburner or recovered by condensation into a liquid form. The condensate may then be separated into organic and water fractions. Thermal desorbers generally operate at relatively low soil discharge temperatures, typically in the range of 150.degree. C. to 500.degree. C. This process can be used for the remediation of a wide range of contaminants such as petroleum products and pesticides.
Steam stripping allows steam to come into contact with the contaminated water causing the contaminants to volatilize and transfer from the water to the steam. Although the contact between the two phases may be achieved by a variety of methods, most traditional units are counter-current packed columns. The driving force for the contaminant transfer is the concentration differential between the liquid and vapour phase. The vapour outlet stream is condensed and the contaminant is recovered in a concentrated water stream. The stripping temperature is important since Henry's law constant (ratio of the contaminant concentration in the water and vapour phase) is temperature dependent. Other important system operating parameters are the steam to feed ratio, and the pH of the feed stream.
Advanced oxidation is a technique that may be used to degrade organic contaminants in ground water and in some cases mineralize them to undetectable levels. Advanced oxidation processes are applicable for treatment of low concentration contaminated ground waters and as a post-treatment step. Ultra-violet light in conjunction with powerful oxidants such as hydrogen peroxide and Fenton's reagent treatment generate powerful hydroxyl radicals. The hydroxyl radicals oxidize the organic contaminants which are kept in an excited state and vulnerable to attack by the UV/oxidant system. In the absence of ultra-violet light, Fenton reagent (dark Fenton's reaction) produces hydroxyl radicals by the interaction of hydrogen peroxide with ferrous salts. The reaction is retarded after complete conversion of the ferrous ions (Fe.sup.2+) to ferric ions (Fe.sup.3+). The irradiation of this solution (photo-Fenton's reaction) includes the photoreduction of Fe.sup.3+ to Fe.sup.2+ ions allowing the generation of hydroxyl radicals to continue. The irradiation of hydrogen peroxide solution involves a single step dissociation of the hydrogen peroxide to form two hydroxyl radicals.